The present invention relates to a transmitting and receiving system in base station for a digital mobile telephone system adopting a time division multiple access system (TDMA) and, more particularly, to a transmission/reception data processing circuit, which permits ready transition from a full to a half rate system without alteration of any hardware but with mere simple alteration of firmware.
By the term "half rate system" is meant a mobile telephone system for speech communication at one half the information transmission rate of the full rate system. According to this half rate system about double capacity of the speech communication is permitted for the full rate system. By way of example, in the Japanese standard digital mobile telephone system, one frame is 40 milliseconds and is divided in time into 6 slots. In the full rate system, two slots in one frame are used for communication, while in the half rate system a single slot in the frame is used for communication.
FIG. 3 shows a prior art time division multiple access system transmission/reception data processing circuit of this type. As shown, the circuit has independent data processing means 126, 128 and 130 and 2-port RAMs 127, 129 and 131 for individual channels and also has multiplexing/separating means 125 for multiplexing and separating data of individual channels, the means 125 being provided between the data processing means 126, 128 and 130 and a radio transmitting/receiving circuit 11.
Such a prior art circuit should have a transmission/reception data processing unit and a 2-port RAM for each channel. When the number of channels to be multiplexed is changed such as in the case of transition from the full rate system to the half rate system, an expensive change of hardware is necessary.
For example, the Japanese standard digital mobile telephone system calls for the full rate system of 3-channel multiplexing to be placed in practical use first and the half rate system of 6-channel multiplexing to be introduced about two years later. According to the full and half rate systems coexist at a single radio frequency. Therefore, if processing units which can accommodate only the 3 channels necessary for the initial full rate system are provided in the prior art structure which requires a data processing unit for each channel, it becomes necessary to replace the transmission/reception data processing units at the time of the future introduction of the half rate system. On the other hand, providing data processing units for 6 channels to accommodate the half rate system from the outset causes a great initial investment.
In the half rate system, the bit rate is one half the bit rate in the full rate system so that about one half the transmission/reception data processing capacity suffices. Therefore, to be compatible with both the half and full rate systems it is necessary to have data processing units having the full rate system processing capacity for about 6 channels, which is also expensive.